The present invention relates to semiconductor devices and their methods of fabrication. More particularly, the present invention relates to the processing of copper interconnect material and the resultant device utilizing the same. Even more particularly, the present invention relates to reducing electromigration in copper interconnect lines by doping their surfaces with a barrier material using wet chemical methods.
Currently, the semiconductor industry is demanding faster and denser devices (e.g., 0.05-xcexcm to 0.25-xcexcm) which implies an ongoing need for low resistance metallization. Such need has sparked research into resistance reduction through the use of barrier metals, stacks, and refractory metals. Despite aluminum""s (Al) adequate resistance, other Al properties render it less desirable as a candidate for these higher density devices, especially with respect to its deposition into plug regions having a high aspect ratio cross-sectional area. Thus, research into the use of copper as an interconnect material has been revisited, copper being advantageous as a superior electrical conductor, providing better wettability, providing adequate electromigration resistance, and permitting lower depositional temperatures. The copper (Cu) interconnect material may be deposited by chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), sputtering, electroless plating, and electrolytic plating.
However, some disadvantages of using Cu as an interconnect material include etching problems, corrosion, and diffusion into silicon.1 These problems have instigated further research into the formulation of barrier materials for preventing electromigration in both Al and Cu interconnect lines. In response to electromigration concerns relating to the fabrication of semiconductor devices particularly having aluminum-copper alloy interconnect lines, the industry has been investigating the use of various barrier materials such as titanium-tungsten (TiW) and titanium nitride (TiN) layers as well as refractory metals such as titanum (Ti), tungsten (W), tantalum (Ta), molybdenum (Mo), and their silicides.2 Although the foregoing materials are adequate for Al interconnects and Alxe2x80x94Cu alloy interconnects, they have not been entirely effective with respect to all-Cu interconnects. Further, though CVD and PECVD have been conventionally used for depositing secondary metal(s) on a primary metal interconnect surface, neither technique provides a cost-effective method of forming a copper-zinc alloy on a Cu interconnect surface. Therefore, a need exists for a low cost and high throughput method of reducing electromigration in copper interconnect lines by decreasing the drift velocity in the copper line/via, in order to decrease the copper migration rate as well as the void formation rate, by using an interim conformal Cu-rich copper-zinc (Cuxe2x80x94Zn) alloy thin film electroplated on a copper (Cu) surface from a stable chemical solution, and controlling the Zn-doping thereof, which improves also interconnect reliability and corrosion resistance.
1Peter Van Zant, Microchip Fabrication: A Practical Guide to Semiconductor Processing, 3rd Ed., p. 397 (1997). 
2Id., at 392. 
Accordingly, the present invention provides a method of reducing electromigration in copper interconnect lines by decreasing the drift velocity in the copper line/via, thereby decreasing the copper migration rate as well as the void formation rate, by using an interim conformal Cu-rich Cuxe2x80x94Zn alloy thin film electroplated on a copper (Cu) surface from a stable chemical solution, and controlling the Zn-doping thereof, which improves also interconnect reliability and corrosion resistance, and a semiconductor device thereby formed. The present method involves electroplating the Cu surface, such as a blanket Cu seed layer and a partial thickness plated Cu layer, in a unique nontoxic aqueous chemical electroplating solution containing salts of zinc (Zn) and copper (Cu), their complexing agents, a pH adjuster, and surfactants, thereby forming an interim Cuxe2x80x94Zn alloy thin film layer having some degree of oxygen (O) concentration, wherein the Zn-doping is controllable by varying the electroplating conditions; and annealing the interim Cuxe2x80x94Zn alloy thin film formed on the Cu surface in an environment such as vacuum, nitrogen (N2), hydrogen (H2), formine (N2H2), or mixtures thereof for reducing the O-concentration in the alloy thin film layer, for modifying the grain structure of the Cuxe2x80x94Zn alloy thin film as well as of the underlying Cu surface, and for forming a mixed Cuxe2x80x94Zn/Cu interface; and further electroplating the alloy thin film layer with Cu for completely filling the via, thereby forming the interconnect structure. The present invention further provides a particular electroplating method which controls the parameters of Zn concentration, pH, temperature, and time in order to form a uniform reduced-oxygen Cuxe2x80x94Zn alloy thin film on a cathode-wafer surface such as a Cu surface for reducing electromigration in the device by decreasing the drift velocity therein which decreases the Cu migration rate in addition to decreasing the void formation rate.
More specifically, the present invention provides a method for fabricating a semiconductor device having an interim conformal Cu-rich Cuxe2x80x94Zn alloy thin film formed on a Cu surface by electroplating the Cu surface in the present chemical solution. The method generally comprises the steps of: (1) providing a semiconductor substrate having a Cu surface; (2) providing a chemical solution; (3) electroplating the Cu surface in the chemical solution, thereby forming an interim Cuxe2x80x94Zn alloy thin film on the Cu surface; (4) rinsing the interim Cuxe2x80x94Zn alloy thin film; (5) drying the interim Cuxe2x80x94Zn alloy thin film; (6) annealing the interim Cuxe2x80x94Zn alloy thin film; (7) filling the via with a Cu-fill on the interim Cuxe2x80x94Zn alloy thin film; and (8) completing fabrication of the semiconductor device.
By electroplating this Cuxe2x80x94Zn alloy thin film on the cathode-wafer surface such as a Cu surface using a stable chemical solution in the prescribed concentration ranges and by subsequently annealing the Cuxe2x80x94Zn alloy thin film electroplated on the Cu surface, the present invention improves Cu interconnect reliability, enhances electromigration resistance, improves corrosion resistance, and reduces manufacturing costs. In particular, the present invention chemical solution is advantageous in that it facilitates formation of an acceptable Cuxe2x80x94Zn alloy thin film over a wide range of bath compositions while the subsequent annealing step removes undesirable oxygen impurities from the formed alloy thin film. The desirable Zn concentration in the Cuxe2x80x94Zn alloy thin film, preferably in a range of approximately 0.2 at. % to approximately 9.0 at. % determined by X-Ray Photoelectron Spectroscopy (XPS) or Auger Electron Spectroscopy (AES), is controllable by varying the electroplating conditions and/or the bath composition. By so controlling the Zn-doping, the present method balances high electromigration performance against low resistivity requirements. Additionally, the Cu surface (e.g., seed layer), being formed by a technique such as electroless deposition, ion metal plasma (IMP), self-ionized plasma (SIP), hollow cathode magnetron (HCM), chemical vapor deposition (CVD), and atomic layer deposition (ALD), is enhanced by the Cuxe2x80x94Zn alloy thin film and is prevented from etching by the high pH value (i.e., basic) of the chemical solution from which the alloy thin film is formed.
Further advantages arise from the present invention""s superior fill-characteristics. The present Cuxe2x80x94Zn electroplating solution facilitates better filling of a Cuxe2x80x94Zn alloy thin film on an interconnect, especially for feature sizes in a dimensional range of approximately 0.2 xcexcm to approximately 0.05 xcexcm, thereby lowering the resistance of the formed Cuxe2x80x94Zn alloy thin film (e.g., in a resistance range of approximately 2.2 xcexcxcexa9xc2x7cm to approximately 2.5 xcexcxcexa9xc2x7cm for approximately 1 at. % Zn content in a Cuxe2x80x94Zn alloy thin film, as deposited). Further, the filling capability is enhanced by three beneficial characteristics of the present invention: (1) the instant chemical solution does not etch copper or a copper alloy seed layer; (2) the introduction of Zn into the alloy thin film as well as onto the Cu interconnect improves both step coverage and nucleation; and (3) a variety of organic additives, such as polyethylene glycol (PEG), organo-disulfides, and organo-chlorides, are compatible and may be included in the instant chemical solution for further enhancing the fill profile and grain structure. The present Cuxe2x80x94Zn electroplating solution provides a desirably low Zn content in a Cu alloy interconnect (e.g., in a concentration range of approximately 0.2 at. % to approximately 1.0 at. %) which also imparts (1) a de minimis increase in resistance as well as (2) a maximum improvement in electromigration resistance. The present chemical solution can also provide a desirably low Zn content (e.g., in a range of  less than  less than approximately 0.1 at. % or  less than  less than approximately 0.2 at. %, virtually undetectable by AES) in a Cu film, wherein the Zn content may be engineered by varying the deposition parameters as well as by modifying the bath composition.